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TEXTBOOK OF BOTANY 



of the later deposits are still unconsolidated, and we can study the 

 plants or parts of plants in peat, marl, and silt deposits. 



The plants that grew a million years ago shed their spores and seeds, 

 their leaves and branches just as modern plants do. Swamp forests were 

 overwhelmed bv catastrophes, and buried in unaerated mud and water 

 and in the delta deposits at the mouths of great rivers just as they are 

 today. Fine "ashes" from violent volcanic eruptions have at various 

 times quicklv surrounded and buried living vegetation. Owing to the 

 poisonous gases and vapors emitted during such eruptions, all living 

 organisms were killed and the plants were often buried under sterile 

 debris. Some of the plants and animals that lived in or fell into bog 

 lakes, where decay is comparativelv slow, have been preserved in peat 

 and coal for centuries. Some of the peat bogs of Siberia have remained 

 frozen since glacial times, and in these natural refrigerators organisms 

 such as mammoths and rhinoceroses have been preserved intact to the 

 present time. 



Both the remains and the various traces of organisms are teraied 

 fossils. Sometimes the material of the plant was carbonized by oxidation- 

 reduction processes which removed most of the oxygen and hydrogen 

 and left beautiful outlines of parts of the plant in carbon (Fig. 355). 

 Molds, casts, and, less commonly, petrified fossils were formed. When 

 ooze surrounds an organism and solidifies, the organism may slowly 

 disintegrate, but a mold of its form is left. If the mold is filled with 



Fig. 355. Carbonized fossils of fern-like leaves in a rock of the carboniferous period. 



Courtesy World Book Co. 



